1. Field of the Invention
This invention relates to the alcoholysis of polyvinyl acetate and more particularly it relates to carrying out such process by a combination of rapid and uniform premixing of catalyst and polyvinyl acetate followed by alcoholysis in a reactor providing a narrow distribution of residence times.
2. Description of the Prior Art
Currently, most partially alcoholyzed polyvinyl alcohol (PAPVA) is manufactured by the "slab-casting" process. In this process a catalyst, e.g. sodium hydroxide, is mixed with the feed solution of polyvinyl acetate in methanol, and the viscous stream is formed into a layer on a moving belt. Reaction time is controlled by the belt speed. Because all the polymer molecules at any given cross-section along the belt have resided on the belt for substantially identical times, the casting belt provides a genuinely "plug-flow" reaction environment, which fosters a narrow distribution of degree of conversion to alcohol. Because the liquid layer is open to the atmosphere, the reaction temperature must be kept below the boiling point of the solvent mixture, which means at or below about 50.degree. C. This results in a relatively low reaction rate. The low rate, coupled with mechanical limitations on belt size, leads to a restricted capacity per line. Reportedly the capacity of a commercial slab casting line is on the order of 15-20.times.10.sup.6 lb/year.
Reportedly, the polymer solids content of the feed to belt-casting alcoholysis lines is restricted to about 35%. Such a low solids level results in a heavier burden on the facilities in which the unreacted methyl alcohol and the by-product methyl acetate are recovered. This restriction on feed solution concentration obtains because of the necessity to avoid overheating, and hence initiation of local reaction during the mixing process. The high rotational speeds, and high shear rates characteristic of catalyst premixing devices of the prior art, coupled with the low reaction temperature characteristic of the slab-casting process, combine to impose a limit on the viscosity of the polyvinyl acetate feed solution that can be processed. Likewise, those premixers of the prior art that function by spinning the feed solution into thin films, or into droplets in order to obtain rapid, intimate contact with catalyst solution also require polyvinyl acetate solutions of relatively low viscosity.
A further consequence of this restriction on feed solution viscosity is that polymers of higher molecular weight must be run at lower feed solution concentrations. This imposes an economic penalty on manufacture of higher molecular weight polyvinyl alcohols.
U.S. Pat. No. 2,642,419 teaches thoroughly mixing polyvinyl acetate solution with an alcoholic solution of sodium hydroxide, partial reacting of the mixture on a moving surface, e.g. a conveyor belt, comminuting the resultant gel, and allowing the comminuted gel to further react. Polyvinyl acetate solution concentrations of 24-30% are cited. Mixing is carried out in a stirred vessel.
U.S. Pat. No. 2,643,994 teaches a continuous process for alkaline alcoholysis of polyvinyl esters. In the process the polymer solution is rapidly and intimately mixed with catalyst solution in a mixing zone (not described), then the mixture is conveyed though a reaction zone of uniform temperature without further mixing, e.g. on a moving belt, and finally the product is comminuted and the catalyst neutralized with a dilute acid solution in an inert solvent. Polymer solutions with viscosities of 1 to 80 poise (100-8000 m.Pa.s) at the reaction temperature are preferred. Reaction times of 32 to 200 min were used to attain conversions ranging from 100 to 56 mole %. No discussion of product properties other than conversion was offered, and the mixing device employed was not disclosed. Mixing times in the mixing device were about 2 minutes. Polymer solution concentrations ranged from 55% to 20%, depending on polymer molecular weight. The reaction temperature was 20.degree. C.
Japanese Patent Application No. 38456/64 (Publication No. 21848/68) discloses saponification reaction carried out in an intermeshing, self-wiping, twin-shaft saponifier device. The mixing and kneading action is provided by eccentrically mounted discs. The polyvinyl acetate feed had a concentration of 10-25% in methanol.
Japanese Patent Application No. 20609/54 (Publication No. 9370/56) discloses mixing the catalyst and the polyvinyl acetate and then feeding and reacting the resultant mixture in a continuous twin-screw saponifier.
Japanese Patent Application No. 15773/58 (Publication No. 13141/60) discloses a continuous saponification vessel having two parallel agitator shafts fitted with stirring blades. The agitators were not self-wiping. Baffle plates were used to reduce by-passing of unreacted polymer. Conversions of 90 to 99 mol % were stated to be attainable.
Japanese Patent Application No. 38457/64 (Publication No. 22,448/67) discloses an intermeshing, self-wiping, twin-screw device used as a continuous saponifier. The purpose of the invention is to prevent adhesion of material to the reactor parts during alcoholysis of polyvinyl acetate. No discussion of operating conditions, or product properties, is presented.
Japanese Patent Application No. 1259/49 (Publication No. 4045/51) claims a continuous apparatus for manufacture of polyvinyl alcohol in which alcoholic solutions of polyvinyl acetate (10-20%) and alkali (13%) are mixed in a high speed mixer (1500-7000 rpm) (not otherwise described) then fed to a screw conveyor type saponifier in which the reaction time is about 5 min. There is no disclosure pertaining to control of conversion.
Japanese Patent Publication No. 19187/65 discloses, as a means for continuous hydrolysis of polyvinyl acetate, a reaction vessel having at least two parallel shafts fitted with lens shaped rotor elements, the rotors having a self-wiping contour. In the example the feed solution contained 40% polyvinyl acetate, the reaction temperature was 35.degree. C., the residence time 40 min. and the conversion was 98 mol %.
U.S. Pat. No. 3,386,981 discloses a method of rapid mixing of viscous and nonviscous liquids, specifically as applied to the production of polyvinyl alcohol. The viscous fluid is formed into a cylindrical, flowing film, and the nonviscous fluid applied as a fine spray to the inner surface of said film. The saponifying mixture is dispersed into droplets by a spinning disc, and the reaction completed in a slurry. Feed solutions of 20% polyvinyl alcohol, reaction temperatures of 50.degree.-90.degree. C., and reaction times of 2 minutes are set forth in the specification. Control of conversion is not mentioned. More complete saponification is stated to result from practice of the invention.
U.S. Pat. No. 3,471,131 discloses a concentric cylinder, rotating device for rapid mixing. Mixing of a 40% polyvinyl acetate-methanol solution with a 2.5% solution of sodium methylate is illustrated in one example. The properties of the product are not described.
U.S. Pat. No. 3,300,460 discloses the combination of mixing polyvinyl acetate and catalyst under very high shear conditions for a very brief period of time, followed by saponification yielding finely divided granular polyvinyl alcohol having high mechanical strength and solubility in hot water. Polyvinyl acetate solutions of 5-30% are disclosed, and temperature rises of 8.degree.-22.degree. C. are cited as typical. The mixing device is described in U.S. Pat. No. 2,687,877.
U.S. Pat. No. 3,072,624 teaches a continuous saponification process, with one object of the invention being to achieve high conversion to polyvinyl alcohol. Polyvinyl acetate and catalyst solutions are mixed in a device of high rotational speed, which generates and then combines thin layers of both components. Polymer solutions of 15% to 30% concentration are preferred. Reaction temperatures are 25.degree. to 60.degree. C. Mixing times of 0.1 min. are cited in the examples. The saponifier which follows the mixer, is in one embodiment, an intermeshing, twin-screw conveyor. The effluent from the saponifier is stated to be a solid mass which is crushed and subdivided, then filtered and dried. Conversions of 99.2-99.5% are cited, at reaction times of 3 to 21 minutes. The patent discloses that in order to obtain a polyvinyl alcohol of uniform quality in a continuous process, it is necessary to use a continuous mixing device which effects a sufficiently uniform mixing in a very short time, and then to transfer the mixture immediately to a saponifying device which can effectively handle the very viscous material as mixing and saponification are continued. There is no teaching of how to control conversion, nor of how to operate the process so as to obtain a narrow distribution of composition.